Zinc oxide particles, which have been used as a sunscreen ultraviolet blocking agent in cosmetic product applications, are ultrafine particles having an average particle diameter of 0.1 μm or less. For example, Patent Documents 1 and 2 disclose zinc oxide fine particles which can be used for ultraviolet blocking agents. However, in the disclosed production methods, particles having a particle diameter of 0.1 μm or more and having both high ultraviolet blocking performance and high transparency cannot be obtained.
On the other hand, some of zinc oxide particles having an average particle diameter of 0.1 μm or more are produced by conventional techniques, but there have been few zinc oxide particles having such a particle diameter and controlled to have a small aspect ratio. Zinc oxide particles having an average particle diameter of 0.1 μm or more and having a large aspect ratio have the disadvantage that high transparency required for sunscreen agents cannot be achieved with stability, and therefore it is desired to develop zinc oxide particles having further excellent properties.
Conventional zinc oxide fine particles having a particle diameter of less than 0.1 μm have the disadvantage that transmitted light increases as particles become smaller, so that the ultraviolet blocking effect at a long wavelength side in a UVA range (wavelength: 315 to 380 nm) is significantly reduced. On the other hand, by increasing the size of zinc oxide particles to 0.1 μm, ultraviolet rays in a UVA range at a longer wavelength side can be blocked as compared to conventional zinc oxide fine particles having a particle diameter of less than 0.1 μm. From such a technical point of view, it is desired to develop zinc oxide particles having a particle diameter of 0.1 μm or more and less than 0.5 μm.
Patent Document 3 describes hexagonal prism- and hexagonal barrel-shaped zinc oxide particles which can be used as a cosmetic. However, in the described production method, hexagonal prism-shaped zinc oxide particles having a particle diameter of less than 0.5 μm cannot be obtained. Further, such zinc oxide particles having a particle diameter of 0.5 μm or more are not preferable because sufficiently high transparency required for sunscreen agents cannot be achieved.
From such a point of view, zinc oxide particles which have a primary particle diameter of 0.1 μm or more and can exhibit higher transparency and higher ultraviolet blocking performance as compared to conventional zinc oxide particles are desired. However, such zinc oxide particles have not been developed.
Zinc oxide particles have a high thermal conductivity, and therefore can be used as a heat releasing filler. As zinc oxide particles having such a property, those having a large particle diameter are often used for increasing the filling rate. However, if the zinc oxide particles can be used in such a manner as to be compounded among particles having a large particle diameter for enhancing thermal conduction as a heat releasing filler, thermal conduction can be more efficiently enhanced.